The operation of nuclear power plants, reprocessing plants research facilities and the use of radioisotopes in industry and diagnostic medicine produces a wide variety of radioactive wastes. Many of these wastes need to be treated in order to reduce the radioisotopes concentration to levels acceptable for discharge to the environment. One of the most conventional processes used for the treatment process is ion exchange.
Nuclear power reactors use fuel rods containing uranium. Cesium is a by product of nuclear fission process of 235U. The spent fuel contains cesium along with several other radioactive elements. These rods are dissolved in concentrated nitric acid and the solution thus obtained is processed for removal of radioactive elements.
It is thus highly desirable to remove cesium from this waste solution to make waste handling easy as well as for possible use of separated cesium as radiation source. These sources can be used for blood irradiation, food preservation, hygeinization of sewage sludge and for radiation sterilization of medical products.
To be useful as a Cesium on exchanger in acidic nuclear waste streams, an inorganic compound must exhibit a number of unique properties. The waste streams to be treated have very high levels of Na, K, Ca, Al, Fe, Zr and H+ concentrations relative to Cs. The candidate ion exchange material must have high Cs capacity in presence of these other cations which are up to five orders of magnitude greater in concentration. The HNO3 concentration in each of the waste streams is ˜3-5 M which demands the exchange material to be stable to acidic and oxidizing environments. The compounds must also be highly resistant to radiolytic degradation and elevated temperatures resulting from decay heat generation. Finally the compound must be amenable to fixation by a suitable binding agent or support that will allow the formation of a bead or grain with good mechanical and hydraulic properties. The binding agent must also possess the same qualities of stability in high acid, oxidizing and radiation environments.
The use of inorganic ion-exchangers and related composites for treating liquid radioactive wastes has recently gained prospect due to greater safety and lower cost of such exchangers in addition to thermal and radiation stability and high selection for the capture of certain ions.
The ion exchange mechanism of AMP or ammonium phosphomolybdate that exists in microcrystalline powder form was first reported by Buchwald and Thistlewaite (1957) and according to Smit and Van (1958), the phosphomolybdate complex ion [PMo12O40]3− consists of a hollow sphere formed by 12 MoO6 octahedra with the PO4 group in the centre of the crystal structure of the ammonium salt of this ion. The ammonium ions with the associated water molecules are probably fitted in between these spheres of negative ions thus accounting for the cohesion of these ions. They also report that the exchange of NH4+ ions for the monovalent ions Na+, K+, Rb+, Cs+, with Cs being much preferred over the others. Thus AMP is a selective ion exchanger for cesium.
AMP is available in fine powder form and hence unsuitable for column operations. To make it suitable for intended practical application like the ones to be used in column operations in nuclear plants involved in handling nuclear wastes, it has to be converted into granular forms for use. To create a composite absorber out of AMP, it must be fixed with a binding agent, substrate or support that will allow it to be used in a packed bed column for the separation of Cesium from highly acidic nuclear wastes with reasonable flow rate and pressure. To improve on the granulometric property, AMP is often mixed with asbestos, paper, silica gel, alumina, macro porous organic resin, polyvinyl acetate or polystyrene etc. These modified AMP containing exchanger could not be used for technological application because of lower amount of active AMP loaded on these support and poor granulomertic property. Because of their disadvantage in possessing unsuitable granulometric and mechanical properties, many methods have been proposed for improving such properties of this inorganic ion-exchanger.
Various ammonium molybdophosphate (NH4)3[(MoO3)12PO4] or AMP bound polymers and polymer composites became well known in the art to separate cesium wherein the polymer and polymer composites comprise of poly methylmethacrylate (PMMA), polyacrylonitrile (PAN) and natural polymers like alginates that showed improvement in granulometric properties.
V, Stoy et. al, in Czech Patent A.O. 181605 achieved improvement in granulomertic properties of the powdered AMP inorganic exchanger in using organic hydrophilic or macro porous polymer and copolymer based on polyacrylonitrile (PAN).
H. Mimura et. al. in Journal of Nuclear Science and Technology, 38, 2001, pp 872-878, teaches the immobilizing ability of prominent biopolymer like aliginic acids and alginates in effective granulation or blending of fine crystals of AMP ion-exchanger that offer a number of advantages such as simplicity of preparation, loading of high content of the active component (AMP), high porosity, high mechanical strength and high acid resistance. However, there remains a technological limitation in recovery of loaded Cesium from this substrate because of its deformation in alkaline media wherein the exchange kinetics is also slow due to rigid polymer/AMP composite structures.
T. J. Tranter et. al, in Advances in Environmental Research 6, 2002, pp 107-121 illustrates polyacrylonitrile matrix immobilized AMP, an engineered form of cesium selective sorbent material but with high equilibrium contact time.
Nilchi et. al, in Applied Radiation and Isotopes, 65, 2007, pp 482-487, teaches the removal of Cs and Co ions from medium active nuclear waste solutions containing granular hexacyanoferrate-based ion exchanger and their PAN (polyacrylonitrile) based composites that were chemically and thermally stable and stable in strong acidic solutions such as ≦5M but the amount of Cs and Co adsorbed decreases with increase in nitric acid concentration. However, PAN (polyacrylonitrile) itself is not stable in required highly acidic and basic conditions (3-5 M HNO3, 1M and above NaOH, 8M HNO3 dissolves PAN). Under these conditions, PAN gets hydrolyzed to polyacrylate and swells thereby increasing bed volume and thus unsuitable for large scale column operations. Due to said swelling and hydrolysis, mechanical properties also reduce wherein said PAN binder is excellent for neutral to weakly acidic solutions.
U.S. Pat. No. 4,714,482 teaches on the formation of thin film polymer blend membranes made by blending organic polymers and inorganic chemicals for gas sensing applications wherein said thin film polymer blend membranes are composited on solid porous beads comprising of polysulfone to impart increased structural strength to the membrane. Moreover, the polymers employed in forming the said thin film polymer blend membranes do not use polysulfone as the polymer in the said blend.
WO 02/35581 discloses a PAN-AMP composite wherein said PAN (polyacrylonitrile) forming the composite is itself not stable in required highly acidic and basic conditions such as 3-5 M HNO3, 1M and above NaOH, 8M HNO3 that dissolves the said PAN wherein under the said conditions PAN gets hydrolyzed to polyacrylate and swells and thus increasing bed volume, and hence undesirable for large scale column operations.
It is thus apparent from the discussions hereinbefore that the granular forms of known polymer-AMP composites of the above mentioned prior arts have technological deficiencies and suffer from one or many of the following drawbacks:                i. hard;        ii. lower available surface area;        ii. low accessibility;        iv. Deformation or swelling in alkaline solutions (acrylates);        v. Decreased adsorption from highly acidic nuclear wastes;        vi. low radiation stability;        vii. slow exchange kinetics (longer equilibrium time)        viii. labor intensive manufacturing and        ix. use of significant quantities of organic solvents, cross-linkers etc.        
Therefore it is imperative to develop and provide for alternatives that would be cost effective in requiring small volume of organic solvent in its process of preparation, has high AMP to polymer loading, is stable to radiation, has fast exchange kinetics (short equilibrium time) and shows increased stability in acidic and alkaline medium with no significant deformation of the polymer structure.